1. Field of the Invention
This invention relates to a pyrolysis furnace for olefin production. More particularly it relates to a pyrolysis furnace suitable for optimizing the thermal cracking reaction of a fluid inside the tubes thereof.
2. Description of the Invention
As to the pyrolysis furnace of hydrocarbons including naphtha, it has been suggested to make the furnace multi-purpose, for example, improvement in not only the yield of ethylene as main product, but also that of propylene as byproduct, or change in the proportion of these two, and also a notable change in the shape of radiant tubes causing the reaction has been brought about.
For example, FIGS. 12 and 13 illustrate a conventional and general structure of the furnace, and radiant tubes 3 are arranged at the lengthwise center of a furnace 2 in the body of a pyrolysis furnace 1. A plurality of burners 4 are provided on the lateral surface of the furnace on both the sides of the tubes, and as shown in FIG. 12, radiant tubes of various types are provided and have been respectively used so as to correspond to their use objects. The type 1 (FIG. 14a) is of the most orthodox tube shape constituting one pass both on the inlet side and on the exit side, and the shapes of the type 2 (FIG. 14b) and the type 3 (FIG. 14c; Japanese patent application laid-open No. 56-93792/1981) based on the type 1 have come to be the recent main types employed. These types are referred to as confluence mode types wherein multiple passes are constituted on the inlet side of the tubes, the respective tubes are joined together at the middle part and one pass is constituted on the exit side thereof. Tubes of relatively small diameter are used at the multiple pass part and after joining, tubes of a large diameter are used to thereby generally equalize the fluid flow rate inside the tubes.
FIG. 15 shows the heat flux distribution along the length of the radiant tubes in the furnace, and it is necessary to raise the fluid temperature of hydrocarbons so as to correspond to this heat flux. FIG. 16 shows the fluid temperature inside the tubes in the tube length direction. From the viewpoint of the reaction inside the tubes, since it is ideal to shorten the retention time of hydrocarbons inside the furance, it is desired to raise the temperature of the fluid at the inlet part of the tubes as soon as possible. Thus this desire is to be directed to line B of FIG. 16. In order to effect this mode, it is necessary that the heat transferability on the inlet side of the tubes is as great as possible. Thus, by making the tubes multiple passes and also making the diameter of the respective tubes smaller, increase in the quantity of heat transfer i.e. the heat flux is obtained.
The type 4 (FIG. 14d) has been referred to as a straight type wherein the tube is of one pass both on the inlet side and on the exit side.
As described above, it is necessary to make the tubes multiple pass tubes and also to make the diameter of the tubes smaller, but if this is applied to types 1 and 4, one cannot help employing an exceedingly severe operation such as feed of a heat flux on a very high level. Thus, taking into account the upper limit of the metal temperature on the exit side, the temperature rise on the inlet side should be suppressed by all means. As a result, types 1 and 4 come to exhibit curve A in FIG. 16. This has undesirable effects on the reaction. Since all tubes have small diameter as far as the exit side, increase in the flow quantity of the fluid accompanying the decomposition reaction makes the pressure loss inside the tubes great.
On the other hand, since types 2 and 3 each employ a constitution of multiple passes of tubes of small diameter only on the inlet side of the tubes the types exhibit curve B in FIG. 16; hence this is ideal as far as the temperature rise of fluid is concerned. However, any tube construction of types 1, 2 and 3 are of hair pin structure having return bends (180.degree. bends); hence the pressure loss at the bend parts occupies a large proportion of the total pressure loss. Thus, since it is necessary to keep the fluid pressure on the exit side of the tube to a definite value, it is necessary in the case of such types to raise the pressure on the inlet side, too, as compared with type 4.
On the other hand, in the olefin formation reaction by pyrolysis, reduction in the hydrocarbon partial pressure inside the tubes more promotes the reaction along with the above temperature distribution. Thus it is better to reduce the pressure loss of the fluid inside the tubes. In this sense, a structure free of such bend is preferred; hence the type 4 is ideal. However, since this makes it impossible to ensure the tube length of the tubes in the aspect of heat transfer, it is necessary to make tubes of small diameter with a multiple pass configuration; hence the above-mentioned problem is still raised. Accordingly, the above arrangement has been employed only in a certain cases, and currently the arrangement has not been widely employed.